1. Field of the Invention
The present invention relates to the field of magnetic memory systems for the storage of binary digital data and in particular to magnetic memory systems that store binary digital data as the presence or absence of cross-tie, Bloch-line pairs in the cross-tie wall of a thin ferromagnetic film.
2. Description of the Prior Art
The propagation of the inverted Neel wall sections in a serial access memory system was proposed by L. J. Schwee in the publication "Proposal On Cross-tie Wall and Bloch-line Propagation in Thin Magnetic Films" IEEE Transactions on Magnetics, MAG 8, No. 3, pages 405-407, September 1972. Such a memory system utilizes a ferromagnetic film of approximately 81% Ni-19% Fe approximately 350 Angstroms (.ANG.) thick in which cross-tie walls can be changed to Neel walls and Neel walls can be changed to cross-tie walls by applying appropriate fields. Associated with the cross-tie wall is a section of inverted Neel wall that is bounded by a cross-tie on one end and a Bloch-line on the other end.
In such a cross-tie wall memory system, information is entered at one end of the serial access memory system by the generation of an inverted Neel wall section, which is formed by a cross-tie on one side and a Bloch-line on the other, that is representative of a stored binary 1 or of a non-inverted Neel wall section (i.e., the absence of a cross-tie and Bloch-line pair) that is representative of a stored binary 0. Such information is moved or propagated along the cross-tie wall by the successive generation (and then the selective annihilation) of inverted Neel wall sections at successive memory cells along the cross-tie wall.
In the D. S. Lo, et al, U.S. Pat. No. 3,906,466 there is disclosed a propagation circuit for the transfer of inverted Neel wall sections at successive memory cells along the cross-tie wall. In the L. J. Schwee U.S. Pat. No. 3,868,660 and in the publication "Cross-tie Memory Simplified by the Use of Serrated Strips," L. J. Schwee, et al, AIP Conference Proceedings, No. 29, 21st Annual Conference on Magnetism and Magnetic Materials, 1975, published April 1976, pages 624-625, and in the publication "Cross-Tie/Bloch-Line Detection," G. J. Cosimini, et al, AIP Conference Proceedings, No. 3, 23rd Annual Conference on Magnetism and Magnetic Materials, 1977, published March 1978, pages 1828-1830, there have been published more recent results of the further development of cross-tie wall memory systems.
In prior art cross-tie wall memory systems, the magnetic film that functions as the storage medium has the property of uniaxial anisotrophy provided by its magnetic field induced easy axis, which easy axis is generated in the magnetic film during its formation in the vapor deposition process. This easy axis provides an anisotropy that constrains the generation of the cross-tie wall along and parallel to the easy axis. In the above L. J. Schwee, et al, AIP publication there are proposed serrated strips of Permalloy film, about 350 Angstroms (.ANG.) in thickness and 10 microns (.mu.m) in width, which serrated strips are etched from a planar layer of the magnetic material so that the strips are aligned along the easy axis of the film. After an external magnetic field is applied normal to the strip length, i.e., transverse the easy axis of the film, the magnetization along the opposing serrated edges rotates back to the nearest direction that is parallel to the edge. This generates two large domains that are separated by a Neel or cross-tie wall that is formed along the centerline of the strip. Cross-ties are energetically more stable at the necks of the serrated edges while Bloch-lines are energetically more stable in the potential wells between adjacent necks.
In the M. C. Paul, et al, U.S. Pat. No. 4,130,888 there is disclosed a cross-tie wall memory system and in particular a data track therefore that is formed of a strip of magnetic material having substantially zero magnetic field induced anisotropy. The data-track-defining-strip of isotropic material utilizes its shape, i.e., its edge contour induced, anisotropy to constrain the cross-tie wall within the planar contour and along the centerline of the film strip. Accordingly, the cross-tie wall is constrained to follow the path defined by the magnetic film strip which path may be configured into a major loop, or circular data track, configuration for large capacity memory storage.
In the M. C. Paul, et al, U.S. Pat. No. 4,153,160 there is provided a nondestructive readout, random access cross-tie wall memory system in which there is provided a shift register having a cross-tie, Bloch-line pair generator at one end and a detector in the middle. The shift register consists of at least 2N memory cells spaced therealong, each memory cell consisting of a store selection and a transfer section. The N cross-tie, Bloch-line pairs, or bits of the to-be-stored N-bit data word, are written or stored into the N memory cells of the shift register between the generator and the detector. For the readout of the stored data word, the N-bits are shifted through the detector into N memory cells (0 through N-1) with the N-1.sup.th bit being left resident in the detector. After readout, the N bits of the readout data word are restored into their original stored memory cells by being reverse shifted through the detector.
However, it has been found that because of the operating characteristics, including the inclusion of defects in the data track or shift register defining strips of thin ferromagnetic material, of the prior art cross-tie wall memory systems, it has been difficult to develop a workable memory system having a capacity of a large number of data bits, e.g., 1K. The primary problem has been with the method of propagation of the data bits, it requiring the successive generation and annihilation of the data bit defining cross-tie, Bloch-line pairs. System tolerances have been found to vary to the degree that random destruction of data bits has made large scale memories unreliable. In the present invention, this primary source of the loss of data bits has been obviated by a static memory system in which the data bits are selectively written into discrete memory elements, one at each intersection of an XY array of discrete memory elements. Each discrete memory element of the array is arranged in a typical two-dimensional, one-bit word memory array.